Anyone willing to play a tune is welcome to Music Mondays at An Sibin, an Irish bar in Inman Square. And so they come, men and women, young and old, in collared shirts, tattoo collars or drag makeup. They trickle in around the 6 p.m. start time, settle into chairs and listen as a featured act performs in a corner by the window. Soon the set will shift from performance to possibility. Guitarists, accordionists, vocalists, even complete newcomers to playing live drift on and off stage.